U.S. Pat. No. 3,502,803 discloses a facsimile graphic communication system wherein the white area on a document or the like is rapidly transported past the scanning station in accordance with the lack of information thereon. White line skipping is provided wherein prescan apparatus and associated stepping devices rapidly step the white or lack of information areas on a document or the like past the optical scanner. Cited in U.S. Pat. No. 3,502,803 is U.S. Pat. No. 3,448,207, which discloses a white line skipping technique for a facsimile system, wherein a document is mounted on a rotating drum arrangement, as opposed to a rotating turret type mechanism as disclosed in U.S. Pat. No. 3,502,803.
In accordance with the present invention, a short coded informational signal is generated which identifies a multiplicity of consecutive white scan lines. Facsimile transmission time is reduced by sending during a signal scan time a code which defines the multiplicity of consecutive white scans instead of transmitting the all white scans individually as normal video. Speed up achieved in all white areas is proportional to the largest number of white lines which can be accommodated by the code. White line skipping is primarily dependent upon "looking ahead" in order to determine that future video lines of data are all white. This is readily accomplished for flat bed scanners with electronic memory. Flat bed scanning involves limited mechanical freedom due to high inertia considerations. Electronic memory provides for the ability to scan, process, store, and transmit on demand without consideration of mechanical limitations.
A drum-based facsimile transceiver has certain mechanical limitations. Primary is that the drum rotation rate cannot be varied. This means that the scanning rate is fixed. Carriage movement must be continuous and lines cannot be rescanned. For variable carriage rates, a variable speed motor, such as a stepper, is required. In order to achieve the required "look ahead" a linear scanner array is required. Thus, when a multiplicity of white lines, in terms of two or more, have been determined by the "look ahead" scanner elements, the scanner can be rapidly advanced during the next revolution and thus effectively skip the white areas. Scanning is then resumed at the next line containing black video or unknown scan line which is immediately beyond the look-ahead array.